mybatis學習（八）——resultMap之association&&collection解析

一、resultMap的使用

resultMap 也是定義返回值類型，返回值為用戶自定義的類型，可用於解決JavaBean中的屬性名和數據庫中的列名不一致的情況

之前對於JavaBean中屬性名和數據庫中的列名不一致的情況，通過有兩種辦法，1、通過在sql中使用別名 2、如果正好符合駝峰命名，需要在settings中配置，現在可以通過resultMap來解決

hotelMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.pjf.mybatis.dao.HotelMapper">
<!-- resultMap定義 
type:javaBean的全類名，
id為該resultMap的唯一標識 -->
<resultMap type="com.pjf.mybatis.po.Hotel" id="myHotel">
<!--id 指定主鍵的封裝規則 
column：數據庫中列名 
property：javaBean的屬性名 -->
<id column="id" property="id" jdbcType="INTEGER" />
<!--result 指定非主鍵的封裝規則
column：數據庫中列名
property：javaBean的屬性名 -->
<result column="hotel_name" property="hotelName" jdbcType="VARCHAR" />
<result column="hotel_address" property="hotelAddress"
jdbcType="VARCHAR" />
<result column="price" property="price" jdbcType="INTEGER" />
</resultMap>

<!-- resultMap使用 -->
<select id="getHotel" resultMap="myHotel">
select * from hotel
where
id=#{id}
</select>
</mapper> 

二、association的使用

association和collection都是用來關聯另一個表的數據，區別就是用來關聯對象的封裝的，而collection是用來關聯集合封裝的，

舉個例子，比如通過查詢酒店，查出該酒店的城市，是一個城市對應一個酒店，用association

而查詢一個城市的酒店，是一對多的，用collection，下面來具體實現下這個例子。

1、環境准備

修改hotel.java代碼，增加一種類成員變量City，通過查詢酒店，直接查出他所在的城市

package com.pjf.mybatis.po;

public class Hotel {
private int id;
private String hotelName;
private String hotelAddress;
private int price;
private City city;

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public String getHotelName() {
return hotelName;
}

public void setHotelName(String hotelName) {
this.hotelName = hotelName;
}

public String getHotelAddress() {
return hotelAddress;
}

public void setHotelAddress(String hotelAddress) {
this.hotelAddress = hotelAddress;
}

public int getPrice() {
return price;
}

public void setPrice(int price) {
this.price = price;
}

public City getCity() {
return city;
}

public void setCity(City city) {
this.city = city;
}

@Override
public String toString() {
return "Hotel [id=" + id + ", hotelName=" + hotelName + ", hotelAddress=" + hotelAddress + ", price=" + price
+ "]";
}
}

增加城市類 City.java

package com.pjf.mybatis.po;

public class City {
private int cityCode;
private String cityName;

public int getCityCode() {
return cityCode;
}

public void setCityCode(int cityCode) {
this.cityCode = cityCode;
}

public String getCityName() {
return cityName;
}

public void setCityName(String cityName) {
this.cityName = cityName;
}

@Override
public String toString() {
return "City [cityCode=" + cityCode + ", cityName=" + cityName + "]";
}
}

還有數據庫的修改，hotel表中增加一列city_code，新增一個city表，

hotel表

 

city表

hotelMapper接口不變

package com.pjf.mybatis.dao;

import com.pjf.mybatis.po.Hotel;

public interface HotelMapper {

    public Hotel getHotel(Integer i);
}

 

 2、association的使用

通過association來關聯city表，使用規則如下

hotelMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.pjf.mybatis.dao.HotelMapper">
    <!-- resultMap定義 type:javaBean的全類名， id為該resultMap的唯一標識 -->
    <resultMap type="com.pjf.mybatis.po.Hotel" id="myHotel">
        <!--id 指定主鍵的封裝規則 column：數據庫中列名 property：javaBean的屬性名 -->
        <id column="id" property="id" jdbcType="INTEGER" />
        <!--result 指定非主鍵的封裝規則 column：數據庫中列名 property：javaBean的屬性名 -->
        <result column="hotel_name" property="hotelName" jdbcType="VARCHAR" />
        <result column="hotel_address" property="hotelAddress"
            jdbcType="VARCHAR" />
        <result column="price" property="price" jdbcType="INTEGER" />
        
        <!--association 關聯的表
               property 指被關聯的類成員變量  
               javaType 指被關聯的類成員變量的全類名   -->
        <association property="city" javaType="com.pjf.mybatis.po.City">
            <id column="city_code" property="cityCode" jdbcType="INTEGER"/>
            <result column="city_name" property="cityName" jdbcType="VARCHAR"/>
        </association>
    </resultMap>

    <!-- resultMap使用 -->
    <select id="getHotel" resultMap="myHotel">
        select
        h.id,h.hotel_name,h.hotel_address,h.price,c.city_code,c.city_name from
        hotel h ,city c
        where
        h.city_code=c.city_code and h.id=#{id}
    </select>
</mapper>

測試類：

package com.pjf.mybatis;

import java.io.IOException;
import java.io.InputStream;
import java.util.Map;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.pjf.mybatis.dao.HotelMapper;
import com.pjf.mybatis.po.Hotel;

public class TestHotel {

    public SqlSessionFactory sqlSessionFactory() throws IOException {
        // mybatis的配置文件
        String resource = "mybatis_config.xml";
        // 使用類加載器加載mybatis的配置文件（它也加載關聯的映射文件）TestHotel.class.getClassLoader()
        InputStream is = Resources.getResourceAsStream(resource);
        // 構建sqlSession的工廠
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
        return sessionFactory;
    }

    // 查
    @Test
    public void getHotel() throws IOException {

        SqlSessionFactory sessionFactory = sqlSessionFactory();
        SqlSession session = sessionFactory.openSession();
        HotelMapper hotelMapper = session.getMapper(HotelMapper.class);
        System.out.println(hotelMapper.getClass());
        Hotel hotel = hotelMapper.getHotel(1004);
        // 打印酒店
        System.out.println(hotel);
        // 打印城市
        System.out.println(hotel.getCity());
        session.close();
    }
}

這時候就可以看到結果了

 3、association的分步查詢的使用

 

三、collection的使用

實例：查詢某個城市的全部酒店

修改city類

package com.pjf.mybatis.po;

import java.util.List;

public class City {
    private int cityCode;
    private String cityName;
    private List<Hotel> hotel;

    public List<Hotel> getHotel() {
        return hotel;
    }

    public void setHotel(List<Hotel> hotel) {
        this.hotel = hotel;
    }

    public int getCityCode() {
        return cityCode;
    }

    public void setCityCode(int cityCode) {
        this.cityCode = cityCode;
    }

    public String getCityName() {
        return cityName;
    }

    public void setCityName(String cityName) {
        this.cityName = cityName;
    }

    @Override
    public String toString() {
        return "City [cityCode=" + cityCode + ", cityName=" + cityName + "]";
    }

}

修改hotelMapper接口（可以重新定義一個接口和mapper.xml文件）

package com.pjf.mybatis.dao;

import com.pjf.mybatis.po.City;

public interface HotelMapper {

    public City  getCityHotel(Integer i);
}

hotelMapper.xml文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.pjf.mybatis.dao.HotelMapper">
    <resultMap type="com.pjf.mybatis.po.City" id="cityHotel">
     <id column="city_code" property="cityCode"/>
     <result column="city_name" property="cityName"/>
     <!--collection被關聯的集合 
           ofType被關聯集合元素的全類名
     -->
     <collection property="hotel" ofType="com.pjf.mybatis.po.Hotel">
        <id column="id" property="id"/>
        <result column="hotel_name" property="hotelName"/>
        <result column="hotel_address" property="hotelAddress"/>
        <result column="price" property="price"/>
     </collection>
    </resultMap>
    
    <select id="getCityHotel" resultMap="cityHotel">
       SELECT c.city_code,c.city_name ,h.id,h.hotel_name,h.hotel_address,h.price 
       FROM city c LEFT JOIN hotel h ON c.city_code=h.city_code 
       WHERE c.city_code=#{cityCode}
    </select>
</mapper>    

測試類

package com.pjf.mybatis;

import java.io.IOException;
import java.io.InputStream;
import java.util.Map;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.pjf.mybatis.dao.HotelMapper;
import com.pjf.mybatis.po.City;
import com.pjf.mybatis.po.Hotel;

public class TestHotel {

    public SqlSessionFactory sqlSessionFactory() throws IOException {
        // mybatis的配置文件
        String resource = "mybatis_config.xml";
        // 使用類加載器加載mybatis的配置文件（它也加載關聯的映射文件）TestHotel.class.getClassLoader()
        InputStream is = Resources.getResourceAsStream(resource);
        // 構建sqlSession的工廠
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
        return sessionFactory;
    }

    // 查
    @Test
    public void getHotel() throws IOException {

        SqlSessionFactory sessionFactory = sqlSessionFactory();
        SqlSession session = sessionFactory.openSession();
        HotelMapper hotelMapper = session.getMapper(HotelMapper.class);
        System.out.println(hotelMapper.getClass());
        City city = hotelMapper.getCityHotel(1);
        // 打印城市
        System.out.println(city);
        // 打印酒店
        for (Hotel  hotel : city.getHotel()) {
            System.out.println(hotel);
        }    
        session.close();
    }
}

查看結果